Is there a website or other source that one (assume non-DoD entity) can inquire and ascertain what companies have bid on a certain requirement?
The short answer is - NO - there is no one source anyone (DoD or non-DOD entity) can access to definitely determine who bid, or will bid, on a particular proposal.
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There is a BUT though - the FedBizOpps.gov website shows all contracts AWARDED, and hence the proposal for which the contract was awarded and winning contractor are shown, but not necessarily all contractors who bid on the original proposal. As part of the link to a contract award at the FedBizOpps website, the contracting activity MAY, but is not required to, post a Bidder's List, and/or information from the Bidder's Conference such as notes, charts, and guest list from which somone might reasonably deduce who bid on the original proposal.